1. Field of the Invention
The present invention relates, in the manufacturing of high-voltage and extra-high voltage extruded cables, to a method and an apparatus for pre-heating the conductor elements of said cables, in particular conductors with metal tape reinforcement, such as for example, Milliken conductors.
The expression “extruded cables” refers to cables wherein the conductor elements are coated with at least one extruded insulating layer. Moreover, in the following and in the attached claims, the term “conductor” shall sometimes be used for the sake of brevity to indicate the “conductor elements” of the cable as a whole.
2. Description of the Related Art
In their most complete form, high-voltage and extra-high voltage extruded cables comprise internal conductor elements made of stranded copper or aluminium wires, an internal semiconducting layer (conductor shield), an insulating layer, an external semiconducting layer (insulation shield), a metal shielding consisting, for example, of helically wound copper strips and/or wires, extruded lead or an aluminium sheet, and optionally an external sheath, made for example of PVC, extruded polyvinyl or other suitable plastics.
The insulating layer, made—as said—by extrusion, is very critical since it is very sensitive to partial discharges that may occur in the presence of defects such as for example, micro-voids and disjunctions between adjoining layers of materials, which may be present in the insulation. These partial discharges accelerate the aging of the insulating material, thus causing its perforation.
Thus, the insulating layer must be as even as possible.
A typical extruded cables manufacturing line comprises a conductor elements pre-heater, a first-stage extruder for the internal semiconductor, a second-stage extruder for the insulation, a third-stage extruder for the external semiconductor and a triple extrusion head for the simultaneous coating of the above layers, a heating and cross-linking tube, and finally, a cooling tube to complete the cross-linking process. Alternatively, it is possible to use more extrusion heads in tandem configuration. Thus, conductors are traditionally pre-heated just before being introduced into the extrusion head for the purpose of reducing the temperature difference between the plastic at the melted state and the conductor on which it is extruded. In fact, such a temperature difference causes the formation of deformations and similar defects onto the contact surface, which—in the final analysis—alter the characteristics of the manufactured cable.
The conductor pre-heating step, moreover, allows obtaining an increase in the plant productivity. In fact, the cable advancement speed must be such as to allow the complete cross-linking of the “insulated core”, expression that in the present description refers to the conductor element, the internal semiconducting layer, the insulation, and the external semiconducting layer as a whole, that is, the conductor element after passing through the extrusion section. The conductor pre-heating reduces the cross-linking times since the conductor releases heat to—or at least does not absorb from—the extruded material, thus preventing the so-called phenomenon of “freezing” of the internal semiconducting layer and of part of the insulation during extrusion. This phenomenon consists in that, without conductor pre-heating, the internal semiconducting layer and the interior of the insulation, when contacting the conductor, release heat to it faster than how they receive heat by conduction from the most external layers, so that they fall below the optimum cross-linking temperature. Thus, during the cross-linking step, there are an external layer being cross-linked, a melted intermediate layer and an internal layer at low temperature. While advancing in the manufacturing line, thanks to the heat received by convection and/or radiation, also the most internal portion, cooled and possibly solidified, is optionally re-melted, brought back to the optimum cross-linking temperature and cross-linked afterwards. As already said, after the cross-linking there is a cooling step, always in radial direction, from the outside inwards.
These changes of state and these temperature changes imply that internal stresses are generated in the insulating layer due to the thermal expansion and contraction, which may worsen the cable performance. As regards the efficiency of the manufacturing line, it is worsened by the fact that the portion that has cooled down or even solidified due to contact with the conductor must be heated again or even re-melted, thus the speed in the cross-linking tube and consequently in the entire plant must be reduced.
Internal stresses, especially in large cables, may cause a worsening in the dielectric properties of the insulating layer.
Document JP 61-271717 describes a plant for making a cable with an insulating resin coating, comprising a feeding drum, driving rollers, a pre-heating device, an extruder of resin on the pre-heated conductor, a cross-linking tube for the extruded resin, driving rollers and a coiler. The pre-heating device, which said document proposes to improve through a device for preventing leakage currents, is based on a system of current induced by an electrical transformer. Thus, the conductor is heated by the heat generated by Joule effect.
Also direct pre-heating techniques—through electrical current—and infrared pre-heating techniques have been proposed.
Nevertheless, the Applicant has noted that the pre-heating techniques mentioned above cannot be satisfactorily applied to conductors provided with metal—in particular copper—tape reinforcement, such as the so-called Milliken conductors. Milliken conductors, and more in general, lobe-section conductors, are widely used for high-voltage cables as they exhibit a lower impedance-resistance ratio with respect to equivalent cables of traditional geometry, and they are not so much affected by the so-called skin effect.
As schematically shown in the cross section of FIG. 1, a Milliken conductor 100 has a plurality of sectors or lobes 101, five lobes 101 being illustrated as an example in FIG. 1, arranged around a core 102. Core 102, made for example of aluminium, has the purpose of supporting lobes 101 eliminating central points thereof. Each lobe 101 in turn consists of a plurality of series of wires 103, 104, . . . , 106, 107. Each series of wires 103-107 is helically wound around the more internal series of wires in the same lobe 101. This multi-lobe geometry forms a substantially circular cross-section of conductor 100, wherein at the junctions between the various lobes 101, however, substantially triangular grooves 108 are formed along the lenght of conductor 100. During extrusion, the extruded material tends to penetrate into said grooves 108, that is to say, it tends to take on an irregular cross-section, not shaped as an annulus (the so-called “fioritura”). If the extrusion occurs at a relatively low pressure, only the internal semiconductor penetrates into the recesses, but if the extrusion pressure is higher, as in the case of triple-head extrusion section, also the insulator penetrates there, thus causing undesired potential gradients in the use of the cable.
To obviate this drawback, in addition to imparting mechanical stability to the conductors, lobe-section conductors—in particular of the Milliken type—are “tape reinforced”, that is to say, they are wound around with a reinforcement tape 109. Said reinforcement tapes, for example, consist of a nylon nonwoven fabric semiconducting layer, a copper layer and another nylon semiconducting layer.
The Applicant has noted that, in the presence of metal tape reinforcement, the metal absorbs most of the heat provided during the conductor pre-heating, whereas the cable remains cold for the Faraday shield principle: the magnetic field lines only concatenate on the reinforcement tape, which shields the conductor arranged internally thereof from the induction current, thus generating a considerable thermal gradient between the conductor core and the reinforcement tape.
Such a thermal gradient is unacceptable since during the cross-linking process, the inner portion of the conductor, which is colder, removes heat from the reinforcement tape and the insulating material, which are hotter, with the onset the above drawbacks.